As a prior art of microwave heating apparatus, a thawing/cooking oven as disclosed in the Japanese Patent Publication No. Sho 55-51541 is well known. Referring to FIG. 23, a prior art thawing/cooking oven comprises a stirrer 3 disposed at the ceiling 2 of a sealable oven body 1, and a magnetron irradiation port 4 disposed in the neighbourhood of the stirrer. Within oven body 1, a detachable food shelf 5 is provided; a detachable liquid tray 6 for water, oil, etc. is provided underneath, in which a foodstuff A may be immersed when necessary; further beneath the tray, a heating means 7 by gas, electricity, etc. is provided. Through a combined work of magnetron irradiation port 4, liquid tray 6 and heating means 7, a heating object is heated with the magnetron irradiation from the above, and at the same time, depending on needs, with steam from boiling water from the underneath.
By the combined use of magnetron irradiation and heating with steam, the time for passing through the zone of maximum ice crystal formation, during which the cell wall is damaged when a frozen foodstuff is thawed, is minimized, and a foodstuff is thawed evenly without allowing escaping of delicious contents. Since water vapour is available, the oven may be used also for thawing frozen breads/frozen cakes, or treating the whole process steps of bread/cake making with fermentation.
Besides heating with steam, the oven is capable of conducting various heating/cooking processes. For example, thawing of frozen pre-processed foodstuffs in fat provided in the liquid tray, thawing of a frozen food package by a combined use of magnetron irradiation and hot air from a heating apparatus (hot air stirred by stirrer in the ceiling), and other cooking methods are disclosed.
In a prior art microwave heating apparatus, however, since the atmosphere in the heating cavity is approximately 100.degree. C. temperature/100% humidity, drawbacks occur such as: when a frozen baked bread or a frozen fried tempura is thawed the surface becomes sticky with steam, affecting the taste; uneven temperature spread is readily caused between the inside and the surface of a foodstuff, which, in a case of thawing frozen breads where the water content is low, gives damage on the stuff affecting the flavor, elasticity or the feeling on teeth.
The issue is explained more in detail. FIG. 24 illustrates the changing temperature of a foodstuff and the oven cavity in a prior art oven wherein the heating with microwave and the heating with steam are conducted at a same time. The temperature of a foodstuff, starting from the frozen temperature (-20.degree. C.), climbs up passing through the zone of maximum ice crystal formation (-1--5.degree. C.) where it consumes a great energy, taking some time there. While a foodstuff is in frozen state it does not absorb the microwave efficiently, instead the microwave goes deep into the foodstuff, and the heat is conducted swiftly. Consequently, the temperature within a foodstuff is relatively even. Application of steam helps the foodstuff quickly pass through the zone of maximum ice crystal formation, but the temperature within the heating cavity becomes approximately 100.degree. C., and the humidity also approximately 100%.
After passing through the zone of maximum ice crystal formation, a foodstuff carries with it those places already thawed and those still frozen. The thawed parts show a dielectric loss several times to several tens of times as large, and microwave is selectively absorbed, which invites uneven temperature within a foodstuff. Especially when steam is applied, the surface of a foodstuff gathers steam, and only a superficial surface of foodstuff is heated by microwave, which expedite the increase of surface temperature. Namely, when the inside temperature of a foodstuff reaches an optimum level, the surface temperature is already far higher than the optimum.
The optimum temperature for a meal is different depending on the kind; it is higher than 80.degree. C. for e.g. steamed meals; 60-70.degree. C. for tempuras, if too hot the food material dehydrates, and moisture is deprived of by coating and taste is affected. The optimum temperature for breads is the room temperature or a temperature slightly higher than bodily temperature; if it is too high the stuff gets damage, and the flavor, elasticity and feeling on teeth are affected. Anyway, the the optimum temperature is at least lower than 90.degree. C.
Also the optimum humidity for a meal is different depending on the kind. For example, the taste deteriorates with both breads and tempuras if their surface get moistened.
As described above, in a prior art microwave heating apparatus, the emphasis has been placed on how swiftly having a heating object pass through the zone of maximum ice crystal formation, while hardly any attention has been paid on how to heat/cook a foodstuff in an environment that is ideal for the foodstuff. Namely, when steam is provided the environment in heating cavity is made to be always at almost 100.degree. C. temperature and approximately 100% humidity, therefore a foodstuff has never been heated/cooked in optimum environment.